Our studies are orientedtowards defining and evaluating the role of proteinases in instigating the inflammatory response. We have isolated and characterized the protienase from whole mouse skin and mouse epidermis and fibroblasts. This proteinase has a unique molecular weight of 9,000 and is a glycoprotein. The proteinase has a distinctive amino acid composition. The enzyme is found in mouse skin and mouse epidermis. Interjection of the enzyme into the peritoneal cavity of mice produces an acute inflammatory response and the accumulation of polymorphonuclear leukocytes. Incubation of the enzyme with the fifth component of complement results in cleavage producing the chemotactic fragment C5A. This enzyme may be important in instigating the inflammatory response in tissue injury.